In recent years, in wiring boards used in the mobile communication field, etc., low-temperature fired boards that can sinter simultaneously with low-resistance conductors such as Ag, Au, Cu, and Pd are used. Also, in the mobile communication field, while individual electronic parts are miniaturized, development of module parts integrated with such parts is advanced. For boards to be used in such module parts, module boards to which a low-temperature fired board is applied are widely used, and individual electronic parts, etc. are mounted on the surface of such a board with a solder. In the mounting step, solder paste printing or mounting of parts is carried out. In general, the mounting is carried out in the mode of a large-sized board for multi-cavity having a number of module boards.
In the foregoing large-sized boards, for the sake of cutting down the manpower of the mounting step, in recent years, it is designed to make the size large so as to realize a square having a side of from 100 mm to 200 mm. For that reason, for the purpose of keeping the registration in the mounting step, dimensional precision or alignment precision of individual module boards is becoming severe. For the sake of attaining such dimensional precision, a shrinkage-free firing method that is not substantially accompanied with shrinkage in the planar direction and in which scattering in size at the time of firing hardly occurs is effective.
Hitherto, as a method of reducing the firing shrinkage of ceramic bodies, there is known a method in which a flexible constraining layer that does not sinter at the firing temperature of the ceramic body is applied, the ceramic body is fired, and the unfired porous constraining layer is removed from the fired ceramic body (see, for example, Japanese Patent No. 2,554,415 (pages 1 to 13 and FIG. 3)). This method involved a problem such that the costs are high because a step of removing the porous constraining layer is necessary after firing.
Also, there is proposed a process of producing a multilayered ceramic board in which in firing a pair of low-temperature firing green sheets made of a different material from each other while putting therebetween a green sheet for repressing shrinkage that does not sinter at the firing temperatures thereof, glass components in the low-temperature firing green sheets are diffused into the green sheet for repressing shrinkage and sintered (see, for example, JP-A-2001-119143 (pages 1 to 10 and FIG. 2)).
According to this process, not only a three-layer structure for forming a composite material comprising a pair of low-temperature firing green sheets having a green sheet for repressing shrinkage put therebetween was always necessary, but also desirable diffusion and sintering of the glass components were not solved yet.